Even in a state in which no incident light is incident on a CCD of a television camera or a video camera, or in a state in which the lens aperture is completely stopped down, it is sometimes the case where a video signal generated as if a faint light were incident on the video signal is outputted. This signal component that is unnecessary in itself is generated mainly due to a dark current of a driving circuit and the like provided inside the CCD, and it is so called the dark shading signal component. As a circuit for executing the dark shading correction by removing this dark shading signal component, there has been provided a dark shading correction circuit.
FIG. 5 is a block diagram showing a construction of a television camera provided with a prior art dark shading correction circuit 500, and the television camera provided with the dark shading correction circuit will be described with reference to the accompanying drawings.
In FIG. 5, a CCD 1, which is a solid-state image pickup device, converts pickup light that is incident via a taking lens (not shown) of the television camera into an electric signal in synchronization with a vertical synchronizing signal VD and a horizontal synchronizing signal HD, and outputs the signal to a preamplifier 2. The preamplifier 2 takes out, for example, RGB video signals of three colors R (Red), G (Green) and B (Blue) in the case of a three-plate type camera by sampling the electric signal outputted from the CCD 1 at a predetermined sampling frequency in synchronization with the vertical synchronizing signal VD and the horizontal synchronizing signal HD and thereafter amplifies the signal to output the video signal after the amplification to an adder 5. In the following signal processing, the signals are processed individually by the three colors R, G and B.
The dark shading correction circuit 500 comprises a sawtooth wave signal generator 3, an amplitude adjuster 4 and the adder 5. The sawtooth wave signal generator 3 is reset in response to the vertical synchronizing signal VD to generate a sawtooth wave signal having the opposite polarity in order to cancel the above-mentioned dark shading signal component, and output the signal to the amplitude adjuster 4. The amplitude adjuster 4 comprises, for example, a pre-set resistor element. By manually changing the resistance value of the pre-set resistor element, the amplitude of the inputted sawtooth wave signal is adjusted, and the sawtooth wave signal after the amplitude adjustment is outputted as a correction sawtooth wave signal. The adder 5 adds the video signal inputted from the preamplifier 2 to the correction sawtooth wave signal inputted from the amplitude adjuster 4 thereby cancelling the dark shading signal component, and outputs the video signal after the correction to a video signal processing circuit 13. The video signal processing circuit 13 executes a gamma processing and a matrix signal processing for conversion of the signal into, for example, an NTSC signal, and outputs the processed video signal to a magnetic recording and reproducing apparatus 14 for recording the signal on, for example, a magnetic tape.
When adjusting the pre-set resistor element of the amplitude adjuster 4, an operator performs adjustments by rotating an adjustment portion of the pre-set resistor element using a screwdriver so that the signal component of the video signal becomes substantially zero while monitoring the video signal outputted from the adder 5 by means of, for example, a CRT display monitor with the lens aperture of the television camera stopped down completely, i.e., so that the dark shading signal component of the video signal is canceled in the manufacturing process of the television camera.
An increase of the dark shading signal component due to a dark current of a driving circuit or the like in the CCD cannot be ignored. The dark shading signal component tends to have an approximate sawtooth waveform synchronized with the vertical synchronizing signal VD. By adding a correction sawtooth wave signal which has the polarity opposite to that of the dark shading signal component and an amplitude approximately identical to the signal component to the video signal outputted from the preamplifier 2, the dark shading signal component can be canceled. That is, when the dark shading signal component outputted from the preamplifier 2 has a waveform approximate to the sawtooth waveform, a video signal which has undergone a sufficient dark shading correction is obtained in the output from the adder 5.
In fact, an actual dark shading correction circuit executes the dark shading correction in the horizontal scanning direction in addition to the above. However, the present invention has no connection with it, and therefore, no description is provided therefor.
When the dark current of the driving circuit or the like in the CCD 1 changes due to, for example, a change in the environmental temperature of the CCD 1, the dark shading signal component outputted from the preamplifier 2 changes. In particular, when the environmental temperature of the CCD 1 is elevated, the dark current of the driving circuit or the like in the CCD 1 increases to increase the dark shading signal component. However, in the above-mentioned prior art dark shading correction circuit, an amplitude adjustment amount of the pre-set resistor element of the amplitude adjuster 4 is fixed after being once adjusted in the manufacturing process as described above, and therefore, it does not follow the possible variation of the dark shading signal component. In particular, when there is a great temperature difference between the current temperature and the temperature at which the amplitude adjuster 4 was adjusted, no sufficient dark shading correction can be effected, and this has caused such a problem that the dark shading signal component disadvantageously remains in the video signal outputted from the adder 5. The remaining dark shading signal component will be referred to as a remaining dark shading signal component hereinafter. When the video signal outputted from the adder 5 is outputted directly to a CRT display or a LCD monitor in the above state, a brightness difference between an upper portion and a lower portion of the screen of the monitor may occur or the display color may vary.
An object of the present invention is to solve the above-mentioned problems and provide a dark shading correction circuit capable of automatically adaptively executing the dark shading correction even when there is a variation of a dark shading signal component due to a change in temperature of the CCD or the like in a television camera or a video camera employing the CCD, thereby always canceling the remaining dark shading signal component.